Single antenna signal retransmission device

ABSTRACT

A device for receiving a signal via an antenna and for retransmitting via the same antenna an augmented signal of identical frequency and coherent phase. A circulator connected to the antenna directs the received signal to a diode switch and thence to a broadband RF amplifier. The switch periodically, alternately isolates and passes the received signal to the amplifier in response to a square wave control signal having a frequency considerably lower than that of the received signal. The sampled, amplified signal is delayed by a time duration substantially equal to one-half the period of the control signal, and then supplied via the circulator to the antenna. The device is useful as an augmentation amplifier, and for enhancing radar target reflectivity.

[ 1 Aug. 15, 1972 SINGLE ANTENNA SIGNAL RETRANSMISSION DEVICE Inventor:Victor F. Cartwright,

Calif.

Assignee: Cartwright Engineering, Inc., Fullerton, Calif.

Filed: April 13, 1970 Appl. No.: 27,538

Fullerton,

Primary Examiner-Malcolm F. Hubler Attorney-Hinderstein & Silber [57]ABSTRACT A device for receiving a signal via an antenna and forretransmitting via the same antenna an augmented signal of identicalfrequency and coherent phase. A circulator connected to the antennadirects the received signal to a diode switch and thence to a broadbandRF amplifier. The switch periodically, alternately isolates and passesthe received signal to the [52] US. Cl ..343/18 B, 343/68 R, 343/18 Damplifier in response to a square wave control Signal [51] Int. Cl..GOls 9/02 having a frequency considerably lower than that of the [58]Field of Search ..34138/68 $8, l6).8llCE, received Signal The Sampled,amplified sign all is 343/18 delayed by a time duration substantiallyequal to onehalf the period of the control signal, and then supplied[56] Reierences Cited via the circulator to the antenna. The device isuseful UNITED STATES PATENTS as an augmentation amplifier, and forenhancing radar target reflectivity. 2,943,318 6/1960 Delorame et al..343/6.8 R 2,703,881 3/1955 Begemann et al ..343/18 E 3,039,089 6/ 1962McMurtrey, Jr ..343/6.8 R 14 Chin's, 6 Drawing Figures 2,412,991 12/1946Labin ..325/25 /0 /6 0/055 R,- 5W/7'5/I 441, 2 0 74-7? 22 /2 J /b a ,5.24

c /4 ,4/vrzxvn/4 /4a 26% 0:24) A //VE SINGLE ANTENNASIGNAL'RETRANSMISSION DEVICE BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to a single antenna signalretransmission device, and more particularly to an augmentationamplifier which transmits via an antenna an amplified signal coherent inphase and identical in frequency with a signal received on the sameantenna.

2. Description of the Prior Art.

For certain applications, it is advantageous to enhance or augment theradar signal returned by a target. For example, if the target is ofsmall physical size or fabricated of a material which exhibits poorradio reflectivity, presence of the target may not be detected since theradar echo is of insufficient amplitude to be picked up by the radarreceiver. By providing an active device on the target to augment theradar reflectivity thereof, the target can be detected at greater rangethan otherwise possible.

In the past, various equipments have been implemented for augmentingtarget reflectivity. Perhaps best known of these is a radar beacon whichincludes a receiver, a modulator and a transmitter. The received radarpulse is delayed by the beacon for a time equivalent to the pulseduration, and then retransmitted at the same frequency but with greateramplitude. While reception and transmission frequencies are identical,the received and transmitted signals are not phase coherent. Moreover,the delay encountered in the beacon introduces a significant range errorin the system. This range error R tc/2, where t is the delay time(equivalent to the pulse duration), and c is the velocity of light. Thiscorresponds to approximately 0.002 usec/foot of distance. For example,if the retransmitted pulse is delayed by 2;.tsec, the range error due totransit time through the beacon is about 1,000 feet.

In other existing radar beacons, the beacon transit time error iseliminated by simultaneously receiving on a first carrier frequency fand transmitting on a different carrier frequency f This condition doesnot simulate a reflected radar signal, and requires the radar receiverto be returned to frequency f,. Of course, there is no phase coherencybetween the input and output RF energy.

Another known technique for achieving electronic enhancement of a radarsignal is to utilize an RF amplifier connected between widely separatedreceiving and transmitting antennas. If the isolation between the twoantennas is greater than the amplifier gain, the system can receive onfrequency f and retransmit on the same frequency f simultaneously andwith phase coherency. However, it is extremely difficult to achieveoverlapping antenna patterns for the receive and transmit antennas,while maintaining the isolation requisite for system operation. Theserequirements make it virtually impossible to install such a system on aphysically small target.

The various shortcomings of the prior art are overcome using theinventive signal retransmission device which facilitates radar signalaugmentation utilizing a single antenna for both reception andtransmission. The received and transmitted signals have the samefrequency and are of coherent phase. Transit time through theaugmentation device is very short, so that insignificant range error isintroduced by the system. If desired, the retransmitted signal may bepulse code modulated.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided apparatus for electronic augmentation of a radar signal. Theapparatus utilizes a single antenna and provides a transmitted signalidentical in frequency and coherent in phase with the received signal.

In a preferred embodiment, the augmentation device comprises a singleantenna connected via a circulator and a diode switch to the input of anRF amplifier. The diode switch operates in response to a square wavecontrol signal to sample a received signal. That is, the switchperiodically, alternately isolates and passes the received signal to theinput of the RF amplifier. The frequency f, of the control signaltypically is considerably lower than the frequency of the received RFsignal.

The sampled signal is amplified by the RF amplifier and supplied via adelay line to the circulator for retransmission by the same antenna.Preferably, the delay line introduces a time delay equal to one-half theperiod of the control signal of frequency f,. As a result, thetransmitted signal increments alternate in time with the received signalincrements or samples passed to the RF amplifier.

Thus it is an object of the present invention to provide an improvedsignal augmentation device.

Another object of the present invention is to provide a signalretransmission system utilizing a single anten- It is another object ofthe present invention to provide a signal retransmission deviceutilizing the same antenna for reception and transmission and whereinthe enhanced, transmitted signal is of identical frequency and in phasecoherence with the received signal.

Still another object of the present invention is to provide anaugmentation device including a single antenna, means for sampling andamplifying the received signal and means for delaying and retransmittingthe amplified sampled signal.

Yet another object of the present invention is to provide a signalaugmentation device including an antenna, a circulator, a diode switchfor periodically, alternately isolating and passing a received signal tothe input of an RF amplifier, and a delay line connecting the output ofthe RF amplifier to the same circulator and antenna, the delay lineproviding a delay related to the periodicity of the switch.

BRIEF DESCRIPTION OF THE DRAWINGS Still other objects, features, andattendant advantages of the present invention will become apparent tothose skilled in the art from a reading of the following detaileddescription of the preferred embodiment constructed in accordancetherewith taken in conjunction with the accompanying drawings whereinlike numerals designate like parts in the several figures and wherein:

FIG. 1 is an electrical block diagram of a single antenna signalretransmission device in accordance with the present invention, and

FIGS. 2a through 2e are waveforms illustrating operation of the deviceof FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,and particularly to FIG. 1 thereof, there is shown a signal transmissiondevice in accordance with the present invention.

Device 10 operates in conjunction with a single anten- 10 na 12 which isconnected to the common terminal 140 of a conventional circulator 14.Device 10 functions to transmit via antenna 12 a signal greater inamplitude, identical in frequency and coherent in phase with a signalreceived on the same antenna 12.

Still referring to FIG. 1, the output port 14b of circulator 14 isconnected to the input of a diode switch 16. Switch 16 also receives viaa line 18 a control signal of frequency f, supplied from a source 20. Asdescribed in detail below, diode switch 16 functions to sample areceived signal and to supply this sampled signal via a line 22 to theinput of an RF amplifier 24. The output of RF amplifier 24 is suppliedvia a line 26 to a delay line 28 or like delay device. The output ofdelay line 28 is supplied via a line 30 to the input port 140 ofcirculator 14.

The control signal supplied by source 20 may comprise a square wave suchas that designated 32 and illustrated in FIG. 2a. Preferably thefrequency f of control signal 32 is considerably lower than thefrequency of a received signal. Very satisfactory system operation isachieved if the frequency of the receiving signal is at least 50 timesthe frequency f, of the control signal supplied on line 18. By way ofexample only, if the device 10 is designated for operation in the Lband, the received signal may lie within the frequency range of from1,200 MHZ to 1,400 MHz, and control signal 32 may have a frequencyf, 20MHz.

If device 10 is to be used for retransmission of pulsed signals, such asthe RF input signal 34 illustrated by the modulation envelope of FIG.2b, the period l/f, of control signal 32 preferably should be somewhatshorter than the duration of the received RF pulse. For example, areceived radar pulse may have a duration of from 1 usec to 10 usec. Iff,20 MHz, the period l/f, of signal 32 is 0.05 usec and the input signalwill be sampled 20 times per usec of received signal pulse duration.

Referring again to FIG. 1, diode switch 16 functions under control ofthe signal on line 18 alternately to isolate and to pass a receivedsignal from circulator port 14b to the input of RF amplifier 24. Asillustrated by FIGS. 2a, 2b and 2c, diode switch 16 passes RF inputsignal 34 when control signal 32 is true. Altemately, when controlsignal 32 is false, diode switch 16 functions to isolate circulatoroutput port 14b from RF amplifier 24. That is, when control signal 32 isfalse, the RF input signal 34 is not passed to RF amplifier 24.Accordingly, the sampled signal on line 22 typically will have theappearance of waveform envelope 36 of FIG. 2c. Although the preferredsample time is one-half the period of the control signal on line 18, asillustrated in FIGS. 2a and 2c, the invention is not so limited, andother duty cycles may be employed.

The sampled signal 36 on line 22 is amplified by RF amplifier 24 toprovide on line 26 a signal having the typical wave shape envelope 38illustrated in FIG. 2d.

RF amplifier 24 may be of the type providing an output signal which iscoherent in phase with the input signal provided thereto. Further,amplifier 24 may be broadband. Thus, for L band operation, RF amplifier24 may be capable of amplifying any signal within the range of from1,200 Ml-Iz through l,400 MHz without tuning or adjustment. Suchamplifiers are available commercially. Of course, the present inventionis by no means limited to operation in this exemplary frequency range.

Delay line 28 preferably exhibits a delay equal to the time duration ofthe sampled signal increments supplied to RF amplifier 24. Thus, in theembodiment described hereinabove wherein switch 16 passes signal 34 forone-half of the period 1 /f, of control signal 32, delay line 28preferably exhibits a delay time of rf, In such an embodiment, ifcontrol signal 32 has a frequency off 20 MHz, delay line 28 may exhibita delay of ref, 0.025 usec. Such a delay readily can be implemented by acoaxial cable of appropriate length.

The output signal from delay line 28 typically will have the appearanceillustrated by wave form envelope 40 of FIG. 2e. Note that signal 40 isidentical in envelope shape, frequency and phase with signal 38 from RFamplifier 24, but is delayed in time by one-half the sample periodthereof. The signal on line 30 is supplied via circulator input port andcommon port 14a for transmission by antenna 12.

As a result of the delay provided by delay line 28, the transmittedsignal increments 40 occur during the time that diode switch 16cisolates circulator port 14b from RF amplifier 24. Accordingly, signalretransmission device 10 effectively alternately receives a signal andretransmits an augmented signal via the same antenna 12. Oscillation isprevented by such time sharing in device 10, and by utilization of aswitch 16 which provides isolation greater than the gain of RF amplifier24.

The augmented signal retransmitted by device 10 is sampled at the ratef,,. However, the RF cycles contained within each increment ofretransmitted signal 40 are coherent and therefore will be additive in afilter or a radar or other receiver. The output of such receiver willappear as though the signal were not sampled, but the received powerwill be one-half that of an input which is not sampled. Thus, whendevice 10 is used as a signal enhancer for radar signals, theimprovement in magnitude of the radar echo will be equivalent to thegain of RF amplifier 24 minus 3 db. For example, if RF amplifier 24exhibits a gain of 43 db, the retransmitted signal power will be 40 dbgreater than that of the signal received by antenna 12. Thus, the signalpower returned to a radar receiver will be 10,000 times that of a signalreflected from a half wave length parasitic reflecting surface. Thiscorresponds to increasing the maximum radar range by a factor of about14.

Since the signal retransmitted by device 10 (FIG. I) is identical infrequency and coherent in phase with the signal received by antenna 12,within the limits of frequency and sensitivity of RF amplifier 24,augmentation device 10 will produce a doppler signal when used with adoppler radar. Further, by modulating or turning on and off RF amplifier24 in response to a digital pulse code supplied thereto, the signalretransmitted by device 10 may be pulse code modulated. Typically, themodulating pulses may have a duration ranging from nsec to 10 msec, eachpulse disabling operation of RF amplifier 24. The resultant pulse codemodulation permits identification at the radar receiver of theparticular target from which the augmented radar return is beingreceived.

While the invention has been described with respect to a preferredphysical embodiment constructed in accordance therewith, it will beapparent to those skilled in the art that various modifications andimprovements may be made without departing from the scope and spirit ofthe invention.

I claim:

1. Apparatus for receiving a signal via an antenna, amplifying saidsignal, and concurrently retransmitting the amplified signal from thesame antenna while retaining the relative amplitude, frequency, andphase characteristics of the received signal, said apparatus comprising:

first means operatively coupled to said antenna for periodicallysampling and amplifying said received signal;

second means operatively coupled to said first means for delaying saidsampled and amplified signal by an amount of time equal to one-half thereciprocal of the sampling rate; and

third means operatively coupled to said second means for coupling tosaid antenna said delayed, sampled and amplified signal.

2. Apparatus as defined in claim 1 wherein said received signal is a cwor pulsed RF signal.

3. Apparatus as defined in claim 1 wherein said first means samples saidreceived signal at a frequency substantially lower than that of saidreceived signal.

4. Apparatus as defined in claim 1 wherein said first means comprises:

an RF amplifier; and

diode switch means for alternately isolating and passing said receivedsignal to said amplifier.

5. Apparatus as defined in claim 4 wherein said diode switch meansprovides an isolation substantially greater than the gain of saidamplifier.

6. Apparatus as defined in claim 4 wherein said diode switch means iscontrolled by a square wave signal of frequency f, substantially lowerthan the frequency of said received signal, and wherein said receivedsignal is passed to said amplifier only when said square wave signal istrue.

7. Apparatus as defined in claim 6 wherein said received signal ispulsed, and wherein said sample period is less than the pulse durationof said received signal.

8. Apparatus as defined in claim 4 further comprising a circulatorcooperating with said antenna and wherein said second means comprises adelay line inserted between the output of said RF amplifier and saidcirculator.

9. Apparatus as defined in claim 8 wherein said delay line comprises acoaxial line exhibiting a delay time approximately equal to hf, where f,is the sampling rate.

10. An RF signal retransmission device comprising:

an antenna,

a circulator connected to said antenna,

an RF amplifier,

switch means for periodically, alternately isolating and passing areceived RF signal from said circulator to the input of said RFamplifier, and a delay line connecting the output of said RF amplifierto said circulator, and providing a delay related to the periodicity ofsaid switch means.

11. A retransmission device as defined in claim 10 wherein said switchmeans comprises a diode switch controlled by a periodic control signalof frequency f,, the frequency of said received signal being at least50f,.

12. A retransmission device as defined in claim 11 wherein said periodiccontrol signal comprises a square wave, said RF signal being passed tosaid amplifier when said square wave is true, and wherein the delay timeof said delay line is substantially equal to the time that said squarewave is true during each period thereof.

13. A retransmission device as defined in claim 10 wherein said RFamplifier is broadband and provides an output signal of the samefrequency and in phase coherence with the input thereto, saidretransmission device thereby transmitting from said antenna anaugmented signal identical in frequency and coherent in phase with thesignal received by the same antenna.

14. A retransmission device as defined in claim 13 further comprisingmeans for pulse code modulating the signal being amplified by said RFamplifier.

1. Apparatus for receiving a signal via an antenna, amplifying saidsignal, and concurrently retransmitting the amplified signal from thesame antenna while retaining the relative amplitude, frequency, andphase characteristics of the received signal, said apparatus comprising:first means operatively coupled to said antenna for periodicallysampling and amplifying said received signal; second means operativelycoupled to said first means for delaying said sampled and amplifiedsignal by an amount of time equal to one-half the reciprocal of thesampling rate; and third means operatively coupled to said second meansfor coupling to said antenna said delayed, sampled and amplified signal.2. Apparatus as defined in claim 1 wherein said received signal is a cwor pulsed RF signal.
 3. Apparatus as defined in claim 1 wherein saidfirst means samples said received signal at a frequency substantiallylower than that of said received signal.
 4. Apparatus as defined inclaim 1 wherein said first means comprises: an RF amplifier; and diodeswitch means for alternately isolating and passing said received signalto said amplifier.
 5. Apparatus as defined in claim 4 wherein said diodeswitch means provides an isolation substantially greater than the gainof said amplifier.
 6. Apparatus as defined in claim 4 wherein said diodeswitch means is controlled by a square wave signal of frequency fssubstantially lower than the frequency of said received signal, andwherein said received signal is passed to said amplifier only when saidsquare wave signal is true.
 7. Apparatus as defined in claim 6 whereinsaid received signal is pulsed, and wherein said sample period is lessthan the pulse duration of said received signal.
 8. Apparatus as definedin claim 4 further comprising a circulator cooperating with said antennaand wherein said second means comprises a delay line inserted betweenthe output of said RF amplifier and said circulator.
 9. Apparatus asdefined in claim 8 wherein said delay line comprises a coaxial lineexhibiting a delay time approximately equal to 1/2 fs where fs is thesampling rate.
 10. An RF signal retransmission device comprising: anantenna, a circulator connected to said antenna, an RF amplifier, switchmeans for periodically, alternately isolating and passing a received RFsignal from said circulator to the input of said RF amplifier, and adelay line connecting the output of said RF amplifier to saidcirculator, and providing a delay related to the periodicity of saidswitch means.
 11. A retransmission device as defined in claim 10 whereinsaid switch means comprises a diode switch controlled by a periodiccontrol signal of frequency fs, the frequency of said received signalbeing at least 50fs.
 12. A retransmission device as defined in claim 11wherein said periodic control signal comprises a square wave, said RFsignal being passed to said amplifier when said square wave is true, andwherein the delay time of said delay line is substantially equal to thetime that said square wave is true during each period thereof.
 13. Aretransmission device as defined in claim 10 wherein said RF amplifieris broadband and provides an output signal of the same frequency and inphase coherence with the input thereto, said retransmission devicethereby transmitting from said antenna an augmented signal identical infrequency and coherent in phase with the signal received by the sameantenna.
 14. A retransmission device as defined in claim 13 furthercomprising means for pulse code modulating the signal being amplified bysaid RF amplifier.